- 36 There are about 200 isotopes of 35 elements that are created by | fission, but many with half lives of seconds or minutes or hours decay rapidly and usually are not detected. An indication is given in Table I of the number of fission products to be expected in samples collected at various times after origin and the percentage contribution of individual fission products to the total activity of all the fission products of common origin. The importance of knowing the age of the fission products when attempting to identify the radioisotopes in a sample is apparent from inspection of the data in Table I. However, if the age is not known a reliable estimate often can be made from calculations of the ratios of certain observed radiolsotopes. Since the ratios are constantly changing, a particular ratio will identify the age of the fission products, providing they are all of the same age and have not been differentially segregated by biological, physical or chemical processes. In the fusion process the nuclei of two atoms are joined together to form the nucleus for one atom, Opposite of the fission process. just the The nuclei of hydrogen atoms are joined to form the nucleus of a helium atom